Approximately sixty percent of the electrical energy in the United States is produced by burning coal, the nation's most abundant energy resource. Renewed emphasis upon the burning of coal for this purpose results from the undesirable dependence on foreign oil and the recent concerns over the early decommissioning of nuclear power plants at forty percent of there intended service life. As new energy demands have focused on the use of coal as a fuel for power generation, the Clean Air Act has strongly influenced the selection of power generation technology. Using coal with high sulfur content requires flue gas desulfurization to reduce the SO.sub.x emission into the atmosphere. Generally, two approaches have been used to mitigate the emission problem. Existing utility-operated pulverized coal combustion boilers are retrofitted with a wet scrubber system using a lime slurry as the SO.sub.x sorbent. This approach produces a calcium sulfite and calcium sulfate sludge--and a disposal problem. A second approach, known as "internal desulfurization," requires the implementation of what is a relatively new technological advance in the electric power industry, fluidized bed combustion.
As a direct response to the energy crisis of the early 1970's, a 6 MMBtu atmospheric fluidized bed combustion unit was fired up in 1974. By 1987, the Fluidized Bed Research Institute of America reported new orders for 54 industrial fluidized bed combustion boilers and 5 utility fluidized bed combustion boilers. Fourteen units were operational in 1986. In a fluidized bed combustion boiler, crushed limestone and coal are fluidized by air pressure. After an initial ignition cycle, the limestone releases CO.sub.2 by its proximity to the ignited coal. As the hot lime (CaO) passes through the ignition zone the SO.sub.x is captured on the surface of the lime particles. The total open porosity of the decarbonated limestone is utilized to convert a substantial amount of SO.sub.x into calcium sulfate (anhydrite). The reactants are eventually used up and the fluidized bed material is deemed to be "spent" and must be replaced. However, after the spent fluidized bed material is removed from the boiler furnace, the issue of its safe disposal must be addressed. A 160 Mw utility combustion boiler consuming 23 tons of coal per hour produces 6.7 tons of spent bed material per hour (160 tons in 24 hours). Due to the characteristics of the raw spent bed material, government regulations require controlled fill licensing and leachate monitoring of landfills to protect the environment from high alkali water contamination.
Thus, although fluidized bed combustion has several advantages, the negative aspects of the fluidized bed combustion technology can be summarized as follows:
1. The available limestone for cement and aggregate production is reduced due to the consumption of a non-renewable natural resource, e.g., at a rate of 207 to 276 tons/day in a 160 MW boiler, to meet emission standards.
2. Disposal sites must be acquired that meet the criteria for filling with spent bed material.
3. Licensing and monitoring of disposal sites is mandated.
4. The availability of future atmospheric fluidized bed combustion development sites is reduced due to negative environmental impact statements.
Thus, it would be desirable to create an environmentally sound and economically beneficial use for spent bed materials created by fluidized bed combustion.
In addition to the above-described drawbacks and problems associated with the use and disposal of spent bed materials from the fluidized bed combustion process, another problematic material created by the power generation industry and by other means is fly ash. Currently, fly ash is under-utilized as a recycled construction material and is typically treated and used as a landfill cap fill material, or in some instances is used as a filler in cement compositions. However, a long felt yet unmet need remains for ways to fully and usefully reuse this power generation byproduct.
It has been suggested that spent bed materials and fly ash can be combined to create a cementitious product. The cementitious potential produced by the combination of spent bed material hydrate, fly ash, and water is derived from the novel source of lime (CaO) in the spent bed materials and its recognized pozzolanic reaction with aluminum oxide (Al.sub.2 O.sub.3) and silicon dioxide (SiO.sub.2)-bearing fly ash. Additional cementitious behavior is associated with the chemical interaction of the sulfur (SO.sub.x) released from coal during combustion, adsorbed by the decarbonated limestone, and subsequently involved with the aluminum oxide in the fly ash. This cementitious behavior is responsible for developing early compressive strength, however, it has been found that such materials exhibit unacceptable levels of expansion over time.
Concrete masonry units utilizing spent bed material and fly ash combined with Portland cement and stabilized road base mixtures are disclosed, respectively, in U.S. Pat. Nos. 4,397,801 and 4,250,134 both to Minnick et al. However, it is known that the compositions disclosed in these patents subsequently yielded unsatisfactory performance. See L. J. Minnick, "Development of Potential Uses for the Residue from Fluidized Bed Combustion Process", U.S., DOE, HCP/10415-55 (March-May, 1981).
The principal hydration products of a blend of spent bed materials and fly ash are gypsum (CaSO.sub.4 --2H.sub.2 O) and calcium sulphoaluminate, the precursor to ettringite "6(CaO).3(SO.sub.3).(Al.sub.2 O.sub.3).31H.sub.2 O". See "Commercial Potential of Atmospheric Fluidized Bed Combustion Concrete Part 2," The Electric Power Research Institute, EPRI GI-7122, Vol. 2, Proj. 2708-4 (January 1991). Further hydration and chemical interaction of the binder after initial set causes the formation of ettringite, which is the principal component of expansive and self-stressing cement (Types K, M and S), which are well known in the art.
Nevertheless, it would be desirable to provide a strategy for developing the full cementing capacity of spent bed material and fly ash while converting the disruptive expansive potential to manageable and desirable levels without the use of energy intensive methods, such as the atmospheric pressure steam treatment disclosed in U.S. Pat. No. 5,100,473--Mitsuda et al.
Accordingly, it is an object of the present invention to provide methods whereby spent bed materials and fly ash can be combined to form a product that is dimensionally stable and that can be used as an aggregate with cement in precast, or poured-in-place concrete application such as masonry or other products.